The present invention relates to a receiver in a radio frequency communications system, and relates more specifically to an even harmonic direct-conversion receiver using an even harmonic quadrature mixer.
Direct conversion reception is one method used to achieve size and weight reductions in a receiver for a radio frequency communications system. Direct conversion reception refers to a reception method in which a received radio frequency signal is frequency converted directly to a baseband signal and then demodulated without an intermediate frequency conversion step. The even harmonic direct-conversion receiver using an even harmonic quadrature mixer has been used to improve deterioration in reception sensitivity. FIG. 12 is a block diagram of an exemplary even harmonic direct-conversion receiver according to the related art.
Shown in FIG. 12 are an antenna 1; a low noise amplifier 2 for amplifying a signal received by the antenna, that is, a radio frequency signal; a bandpass filter 3; and an even harmonic quadrature mixer 4 comprising two unit even harmonic mixers. A local oscillation signal output from local oscillator 5 and the received signal (of frequency frf) passed through bandpass filter 3 are input to the even harmonic quadrature mixer 4. It is to be noted that the frequency (fp) of the local oscillation signal output from the local oscillator 5 is approximately one-half the frequency of the received signal, and the even harmonic quadrature mixer outputs a baseband signal (of frequency |frfxe2x88x922fp|) by mixing the received signal and the local oscillation signal. Also shown in FIG. 12 are a low pass filter 6; a baseband signal amplifying circuit 7 comprising a plurality of baseband signal amplifiers for amplifying the baseband signal; and a signal processing circuit 11. The signal processing circuit 11 comprises an A/D converter 8, digital filter 9, and a digital operating circuit 10.
Operation of this even harmonic direct-conversion receiver is described next.
Referring to FIG. 12, a signal received at antenna 1 is amplified by low noise amplifier 2, and noise components at frequencies outside the received frequency band are strongly suppressed by the bandpass filter 3. The received signal passed by the bandpass filter 3 is then mixed with the output signal from the local oscillator 5 by the even harmonic quadrature mixer 4 (see FIG. 14) comprising two unit even harmonic mixers. The even harmonic quadrature mixer 4 then outputs I channel and Q channel baseband signals. The frequency of the oscillation wave output by the local oscillator 5 is approximately half the frequency of the signal received by the antenna 1.
The I and Q channel baseband signals output from the even harmonic quadrature mixer 4 are filtered by the low pass filter 6 to remove extraneous signals other than baseband signals. The baseband signals filtered by the low pass filter 6 are amplified to a level appropriate for input to the A/D converter 8 by the baseband signal amplifying circuit 7, which comprises a plurality of baseband amplifiers. The amplified baseband signals are converted to digital signals by the A/D converter 8, passed through the digital filter 9, and demodulated data is then obtained by the digital operating circuit 10.
It is possible to suppress deterioration in reception sensitivity due to secondary intermodulation distortion, autodetection of the local oscillation wave, and other mixing of even harmonics by using an even harmonic quadrature mixer in an even harmonic direct-conversion receiver as described above. In addition to enabling receiver downsizing by use of a direct-conversion design, it is therefore also possible to improve reception sensitivity by using an even harmonic quadrature mixer.
It should be noted that the local oscillator 5 outputs to the even harmonic quadrature mixer 4 a local oscillation wave at approximately half the received signal frequency. At the same time, a radiation wave of substantially the same frequency as the local oscillation wave is also emitted from the local oscillator 5. This is referred to below as the radiation wave. When this radiation wave is input from the antenna 1 or input terminal of the low noise amplifier 2, the radiation wave from the local oscillator 5 is input to the low noise amplifier 2 in addition to the received signal.
Due to the nonlinearity of the amplifier, the low noise amplifier 2 thus outputs the received signal, the radiation wave, and a harmonic component of each. The second harmonic of the radiation wave is substantially the same frequency as the received signal. It therefore cannot be removed by the bandpass filter 3 disposed downstream, and is thus input to the even harmonic quadrature mixer 4. As a result, a dc offset voltage caused by the radiation wave is contained in the baseband signals output from the even harmonic quadrature mixer 4.
While the dc offset voltage contained in the baseband signals output from the even harmonic quadrature mixer 4 are relatively small, the dc offset voltage input to the A/D converter 8 becomes extremely high due to the extremely high gain of the baseband signal amplifying circuit 7. This then becomes a factor in data recognition errors when the data is subsequently evaluated by the digital operating circuit 10, and the bit error rate deteriorates.
FIG. 13 shows another even harmonic direct-conversion receiver using an even harmonic quadrature mixer according to a second example of the related art. It should be noted that like parts are identified by like reference numeral in FIG. 12 and FIG. 13, and further description thereof is thus omitted below.
Shown in FIG. 13 are a first low noise amplifier 2a, and a second low noise amplifier 2b disposed between the first low noise amplifier 2a and bandpass filter 3. A signal received from the antenna 1 is amplified by the first low noise amplifier 2a and second low noise amplifier 2b. The amplified signal is then input to the even harmonic quadrature mixer 4, and then demodulated to the original data by the downstream signal processing circuit 11.
A receiver thus comprised to amplify a signal received from the antenna using a plurality of low noise amplifiers for input to the even harmonic quadrature mixer is used, for example, in mobile communications base station equipment, and is used to amplify extremely weak received waves to an input level appropriate to a downstream even harmonic quadrature mixer while also maintaining a sufficient NF.
It is also possible for a radiation wave to be input from antenna 1 or the input terminals of first low noise amplifier 2a or second low noise amplifier 2b with a configuration as shown in FIG. 13. When a radiation wave is input from second low noise amplifier 2b, a drop in reception sensitivity can be expected similarly to that in the first prior art example described above. If a radiation wave is input before the first low noise amplifier 2a, a second harmonic component of the local oscillation wave (radiation wave) will be contained in the components of the received signal amplified by the first low noise amplifier 2a. As a result, the dc offset voltage will increase as the signal is subsequently amplified by the second low noise amplifier 2b and baseband signal amplifying circuit 7. This then becomes a factor in data recognition errors when the data is subsequently evaluated by the downstream signal processing circuit 11, and the bit error rate deteriorates.
It should be noted that to achieve a desired NF through amplification of a weak received signal to a desired signal level by a plurality of low noise amplifiers, the gain of the first low noise amplifier 2a is typically not very high and the second low noise amplifier 2b is set to a high gain level. If a particularly strong radiation wave is input to the first low noise amplifier 2a in this case, the received signal will be masked at the output terminal of the amplifier by the second harmonic of the local oscillation wave (radiation wave), and a large drop in reception sensitivity occurs.
It will thus be known from the above description that a conventional even harmonic direct-conversion receiver does not comprise a means for preventing radiation wave input from the input terminal of a low noise amplifier. As a result, a radiation wave input from an input terminal of a low noise amplifier produces a dc offset voltage and becomes a factor contributing to a loss of reception sensitivity.
Furthermore, a conventional even harmonic direct-conversion receiver having a plurality of low noise amplifiers also does not comprise a means for removing a radiation wave input from an input terminal of a low noise amplifier. Therefore, if a radiation wave input to the first low noise amplifier is amplified together with the received signal, it is not possible at a next low noise amplifier to remove the second harmonic of this local oscillation wave (radiation wave) from the received signal. This becomes a factor contributing to a loss of reception sensitivity.
Therefore, with consideration for the above described problems, a first object of the present invention is to provide an even harmonic direct-conversion receiver comprising a means for preventing radiation wave input to a low noise amplifier.
Furthermore, a second object of the present invention is to provide an even harmonic direct-conversion receiver comprising a means for removing a radiation wave input to a low noise amplifier.
Yet further, a third object of the present invention is to provide a communications system comprising a mobile communications terminal having a direct-conversion receiver and a mobile communications base station.
An even harmonic direct-conversion receiver according to the present invention comprises: an antenna; an amplifier for amplifying a received wave received by said antenna; a local oscillator for generating a local oscillation wave at approximately half the received wave frequency; an even harmonic quadrature mixer for mixing a received signal output from said amplifier and a local oscillation wave output from said local oscillator, and generating a baseband signal; a signal processing circuit for demodulating a baseband signal output from said even harmonic quadrature mixer; and a filter means disposed before said amplifier for preventing input of a radiation wave emitted from said local oscillator to said amplifier by passing the received wave received by said antenna and suppressing a frequency band at approximately half the received wave frequency.
An even harmonic direct-conversion receiver according to the present invention preferably comprises a bandpass filter as the filter means.
Alternatively, an even harmonic direct-conversion receiver according to the present invention preferably comprises a band-elimination filter as the filter means.
Alternatively, an even harmonic direct-conversion receiver according to the present invention preferably comprises a high pass filter as the filter means.
Furthermore, an even harmonic direct-conversion receiver according to the present invention preferably comprises an amplifier having a filter circuit for eliminating from said received signal a local oscillation wave emitted from the local oscillator and input to the amplifier.
A further even harmonic direct-conversion receiver according to the present invention comprises: an antenna; a first amplifier for amplifying a received wave received by said antenna; a second amplifier for amplifying output of said first amplifier; a local oscillator for generating a local oscillation wave at approximately half the received wave frequency; an even harmonic quadrature mixer for mixing a received signal output from said second amplifier and a local oscillation wave output from said local oscillator, and generating a baseband signal; a signal processing circuit for demodulating a baseband signal output from said even harmonic quadrature mixer; and a filter means disposed before said first amplifier or said second amplifier for preventing input of a radiation wave emitted from said local oscillator to said first amplifier or said second amplifier by passing the received wave received by said antenna and suppressing a frequency band at approximately half the received wave frequency.
This even harmonic direct-conversion receiver according to the present invention preferably comprises a bandpass filter as the filter means.
Alternatively, this even harmonic direct-conversion receiver according to the present invention preferably comprises a band-elimination filter as the filter means.
Alternatively, this even harmonic direct-conversion receiver according to the present invention preferably comprises a high pass filter as the filter means.
Furthermore, in this even harmonic direct-conversion receiver according to the present invention the first or second amplifier preferably comprises an amplifier having a filter circuit for eliminating from said received signal a local oscillation wave emitted from the local oscillator and input to the amplifier.
A transmitting and receiving apparatus according to the present invention comprises: an antenna; an even harmonic direct-conversion receiver having an amplifier for amplifying a received wave received by said antenna, a local oscillator for generating a local oscillation wave at approximately half the received wave frequency, an even harmonic quadrature mixer for mixing a received signal output from said amplifier and a local oscillation wave output from said local oscillator and generating a baseband signal, a signal processing circuit for demodulating a baseband signal output from said even harmonic quadrature mixer, and a filter means disposed before said amplifier for preventing input of a radiation wave emitted from said local oscillator to said amplifier by passing the received wave received by said antenna and suppressing a frequency band at approximately half the received wave frequency; and a transmitter having a signal processing section for processing a signal output from said even harmonic direct-conversion receiver and outputting a baseband signal for transmission, a signal processing circuit for modulating the baseband signal for transmission output from said signal processing section, a local oscillator for generating a local oscillation wave at a frequency of approximately half the transmission signal frequency, and an even harmonic quadrature mixer for mixing a local oscillation wave output by said local oscillator and a baseband signal for transmission modulated by said signal processing circuit and outputting a transmission signal.
In a further transmitting and receiving apparatus according to the present invention the even harmonic direct-conversion receiver comprises: a first amplifier for amplifying a received wave received by said antenna; a second amplifier for amplifying output of said first amplifier; a local oscillator for generating a local oscillation wave at approximately half the received wave frequency; an even harmonic quadrature mixer for mixing a received signal output from said second amplifier and a local oscillation wave output from said local oscillator, and generating a baseband signal; a signal processing circuit for demodulating a baseband signal output from said even harmonic quadrature mixer; and a filter means disposed before said first amplifier or said second amplifier for preventing input of a radiation wave emitted from said local oscillator to said first amplifier or said second amplifier by passing the received wave received by said antenna and suppressing a local oscillation wave of a frequency approximately half the received wave frequency.